Directional coupler with arbitrary even and odd mode impedance product for power monitoring applications

This paper presents a new approach towards the design and use of directional couplers for power monitoring in RF front-end circuits. A conventional power monitoring circuit comprises a weakly coupled directional coupler and an attenuator at the coupled port where the attenuator desensitises the coupler from the effects of a mismatch at the coupled port. In a conventional design, the dimensions of the coupled lines are chosen so that the product of the even and odd mode impedances is equal to the square of the reference impedance; this condition ensures good matching of the coupler at all ports. An alternative approach to achieve high directivity is presented where the product of the impedances terminating the direct port and the coupled port is adjusted to match the product of the even and odd mode impedances of the coupled lines. For applications demanding 50 Ohm terminations at all ports, impedance transformation is employed. Measurements of fabricated directional couplers are presented showing improvements in directivity which are available through this new design approach. Specifically, it is shown that a directional coupler fabricated in a multilayer LTCC substrate offers improved directivity when terminated into a lower impedance at the coupled port. Thus, a capability to correct systematic errors in the fabrication process of directional couplers is provided. Furthermore, the freedom to choose lower terminating impedances at the design stage opens up the potential to produce RF couplers which make use of the capability to fabricate closely spaced transmission lines in high precision production processes such as photo lithography.